1. Field of the Invention
This invention relates to an optical recording medium and an optical recording method, and more particularly to an optical recording medium having a plurality of stacked rewritable recording layers and an optical recording method for recording information on said optical recording medium.
2. Description of the Related Art
FIG. 1 shows a conventional optical recording medium. As shown in FIG. 1, the optical recording medium has a PC (polycarbonate) substrate 10, a first recording layer 11, an intermediate layer 12, a second recording layer 13 and a cover layer 14 stacked sequentially from a backside of an incidence plane of laser beam L.
Each of the first recording layer 11 and the second recording layer 13 is a rewritable layer on which a phase change layer is stacked. With use of the laser beam L irradiation, the phase change layer is heated then cooled to change its phase. The first recording layer 11 has a thick metal reflective layer stacked for improving a reflectivity against the laser beam L to gain superior rewrite characteristics. On the other hand, since the laser beam L is irradiated to the first recording layer 11 through the second recording layer 13, the second recording layer 13 has no reflective layer or a very thin reflective layer.
Therefore, if information is recorded on the first recording layer 11 and the second recording layer 13 with a same recording density, rewrite characteristics of the second recording layer 13 is inferior to rewrite characteristics of the first recording layer 11. In spite of this fact, if a recording method of recording with a same recording density on each recording layer in the same way as DVD9 (dual layer DVD-ROM) which is now in practical use, it is necessary to reduce the recording density of the first recording layer 11 corresponding to the inferior rewrite characteristics of the second recording layer 13. However, recording capacity is decreased corresponding to the reduction of the recording density.
Number of the layers composing the optical recording medium as above described is not limited to two as shown in FIG. 1. Other optical recording medium composed of three layers, four layers or the like is also acceptable. In these cases, a thick metal reflective layer is stacked at a recording layer at the most distant part from the incidence plane, but no reflective layer or a very thin reflective layer is stacked at a recording layer at the incidence plane side. Therefore, the same problem is created in the optical recording medium composed of three layers, four layers or the like.